Carbon nanotubes are wires of pure carbon with nanoscale dimensions. The diameter of a single-walled carbon nanotube (SWNT) is typically in the range of approximately 1-5 nm. SWNTs generally consist of a single atomic layer thick sheet of graphite configured into a cylinder. Multi-walled carbon nanotubes (MWNT) generally consist of a plurality of concentric nanotube shells and have a diameter generally on the order of about 50 nm. Nanotubes have potential applications in a wide variety of formats including electronics, materials, biotechnology and the like.
There are currently three general methods for the synthesis of SWNTs: arc discharge, laser ablation, and chemical vapor deposition (CVD) system. In the arc discharge method, an electric arc discharge is created between two carbon electrodes either with or without a catalyst present. Nanotubes are self-assembled from the resulting carbon vapor. The method is a fast method which produces a large amount of impure nanotube material. In laser ablation, a high power laser beam is directed onto a graphite target. Typically, the graphite target is a volume of carbon-containing feedstock gas such as methane or carbon monoxide. Laser ablation generally produces a small amount of clean nanotubes.
In chemical vapor deposition, a substrate, such as silicon, is prepared by sputtering or otherwise patterning a metal layer onto the substrate. Chemical etching or thermal annealing is then used to create wells in the substrate that are used to induce catalyst particle nucleation. Next, during the nanotube synthesis phase, an energy source transfers energy to a gaseous carbon molecule to put the molecule into the gas phase. Methane, carbon monoxide, ethylene, or acetylene is generally used as the carbon source. The transfer energy acts to split the carbon source molecule into a reactive atomic carbon. Nanotubes are formed as the atomic carbon diffuses towards the substrate and binds with the metal catalyst. CVD is the considered the easiest of the three methods to scale up for commercial applications. In addition, as compared to the other two methods, CVD has the advantage that the nanotube catalyst structures used to initiate growth can be defined lithographically.
Some current methods for creating long SWNTs in a CVD reaction chamber require the use of two or more furnaces. Other methods exist for the synthesis of arrays of long SWNTs such as that disclosed by Liu et al. in United States Patent Application Publication No. 2005/0112051. The Liu group has described the synthesis of long SWNTs using a technique based on “fast heating.” The Liu process generally involves heating the nanotube catalyst and substrate to a temperature of between 850 and 1050° C. for 10 to 20 minutes. Since the process includes a cumbersome heating step, the process is less efficient than a method of synthesis that does not require fast heating. In addition, the Liu group method requires a post processing step in order to add electrical contacts.
Therefore, the need exists for an efficient system and method for the creation of ultra-long arrays of nanotubes and nanotube electrodes.